Method and apparatus for alerting a pilot to a hazardous condition during approach to land

ABSTRACT

A terrain-awareness system (TAS) provides LOOK-AHEAD/LOOK-DOWN as well as LOOK-UP terrain advisory and warning indications to the pilot of an aircraft of a hazardous flight condition. The TAS includes an airport data base as well as a terrain data base that is structured to provide various resolutions depending on the topography of the particular geographic area of interest. Navigational data from a satellite-based navigational system, such as a global positioning system (GPS), is used to provide a LOOK-AHEAD/LOOK-DOWN and LOOK-UP terrain advisory and terrain warning indications based upon the current position and projected flight path of the aircraft. Since the terrain advisory and the warning signals are a function of the flight path of the aircraft, nuisance warnings are minimized.

This application is a continuation of Ser. No. 08/509,642 filed Jul. 31,1995 now U.S. Pat. No. 5,839,080.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a terrain awareness system (TAS) andmore particularly to a system for alerting a pilot of an aircraft of adangerous flight condition which monitors the position as well as thetrajectory of an aircraft based upon a satellite-based navigationsystem, such as a global positioning system (GPS), to provide aLOOK-AHEAD/LOOK-DOWN as well as LOOK-UP terrain advisory and warningindications based upon stored terrain data which provides relativelylonger warning times than known ground proximity warning systems whileminimizing nuisance warnings.

2. Description of the Prior Art

Various systems are known in the art that provide warnings and advisoryindications of hazardous flight conditions. Among such systems aresystems generally known as ground proximity warning systems (GPWS) whichmonitor the flight conditions of an aircraft and provide a warning ifflight conditions are such that inadvertent contact with terrain isimminent. Among the flight conditions normally monitored by such systemsare radio altitude and rate, barometric altitude and rate, air speed,flap and gear positions. These parameters are monitored and an advisorysignal and/or warning signal is generated when the relationship betweenthe parameters is such that terrain impact is likely to occur. Typicalexamples of such systems are disclosed in U.S. Pat. Nos. 3,715,718;3,936,796; 3,958,218; 3,944,968; 3,947,808; 3,947,810; 3,934,221;3,958,219; 3,925,751; 3,934,222; 4,060,793; 4,030,065; 4,215,334; and4,319,218, all assigned to the same assignee as the assignee of thepresent invention and hereby incorporated by reference.

While the above-referenced systems do provide advisory and warningsignals in the event of proximity to terrain, the warnings generated bysuch systems are based solely upon flight conditions of the aircraft anddo not utilize any navigational information. Consequently, thesensitivity of such systems must be adjusted to provide adequatewarnings when a hazardous flight condition exists without generatingfalse or spurious warnings. However, such an adjustment can result in acompromise that may still result in nuisance warnings over terrainunique to particular geographic areas and shorter than desired warningtimes in yet other geographic areas.

Several attempts have been made to improve upon such ground proximitywarning systems utilizing ground-based navigational information. Forexample, U.S. Pat. Nos. 4,567,483; 4,646,244; 4,675,823; and 4,914,436all disclose ground proximity warning systems which monitor the positionof the aircraft relative to stored terrain data in order to providemodified ground proximity warnings. However, the utility of such systemsis limited. For example, the systems disclosed in U.S. Pat. Nos.4,567,483 and 4,914,436 disclose ground proximity warning systems whichutilize navigational data to modify predetermined warning envelopessurrounding certain particular airports.

U.S. Pat. Nos. 4,646,244 and 4,675,823 disclose terrain advisory systemswhich utilize various ground-based navigational inputs and storedterrain data to provide various ground proximity warning systems basedon the position of the aircraft. In order to conserve memory, theterrain data is modeled in various geometric shapes as a function of theelevation of the terrain within the area defined by the geometric shape.The warning signals are generated as a function of the position of theaircraft relative to the model. While such systems do provide adequateterrain warnings, such systems may cause nuisance warnings under certainconditions since the predicted trajectory of the aircraft is not takeninto consideration.

Another problem with such systems is that they are based uponground-based navigational systems, such as VOR/DME and LORAN. Suchground-based navigational systems are being replaced with relativelymore accurate satellite systems, such as the global positioning system(GPS). Moreover, such ground-based systems are only able to providetwo-dimensional position data. Thus, with such systems, the altitude ofthe aircraft must be supplied by an auxiliary device, such as analtimeter, which increases the complexity of the system as-well as thecost.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve various problemsassociated with the prior art.

It is yet another object of the present invention to provide a terrainawareness system which provides increased warning times to the pilot ofan aircraft of a hazardous flight condition while minimizing nuisancewarnings.

It is yet another object of the present invention to provide a systemwhich provides a LOOK-AHEAD/LOOK-DOWN as well as a LOOK-UP terrainadvisory and warning indications to the pilot of an aircraft of ahazardous flight condition based upon the predicted trajectory of theaircraft.

It is yet another object of the present invention to provide a terrainawareness system which utilizes inputs from a satellite-based navigationsystem, such as the global positioning system (GPS).

Briefly, the present invention relates to a terrain awareness system(TAS) which provides LOOK-AHEAD/LOOK-DOWN as well as LOOK-UP terrainadvisory and warning indications to the pilot of an aircraft of ahazardous flight condition. The TAS includes an airport data base aswell as a terrain data base that is structured to provide variousresolutions depending on the topography of the particular geographicarea of interest. Navigational data from a satellite-based navigationsystem, such as a global positioning system (GPS), is used to provide aLOOK-AHEAD/LOOK-DOWN and LOOK-UP terrain advisory and terrain warningindications based upon the current position and projected flight path ofthe aircraft. Since the terrain advisory and the warning signals are afunction of the flight path of the aircraft,,nuisance warnings areminimized.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects of the present invention will be readilyapparent with reference to the following specification and attacheddrawings, wherein:

FIGS. 1A and 1B represent a block diagram of a terrain awareness system(TAS) in accordance with the present invention;

FIG. 2 is a diagram of the world broken down into various latitudesegments and longitude segments in accordance with the presentinvention;

FIG. 3 is a graphical representation of the various memory map andresolution sequencing in accordance with the present invention;

FIG. 4A is an exemplary representation of the digital header andsubsquare mask word for identifying geographical areas in accordancewith the present invention;

FIG. 4B is a terrain map alternative to FIG. 3;

FIG. 5 is a graphical illustration of the LOOK-AHEAD distance assumingthe aircraft turns at a 30° angle in accordance with the presentinvention;

FIG. 6 is a graphical representation of a ΔH terrain floor boundarywhich forms the bases for a terrain advisory signal and a terrainwarning signal in accordance with the present invention;

FIG. 7 is a graphical illustration of a terrain advisory signal for anaircraft relative to the terrain and an airport for a condition when theaircraft flight path angle is less than a first predetermined referenceplane or datum in accordance with the present invention;

FIG. 8 is similar to FIG. 7 but for a condition when the aircraft flightpath angle is greater than the first predetermined reference plane;

FIG. 9 is a graphical illustration of a terrain warning signal for anaircraft relative to the terrain and an airport for a condition when theaircraft flight path angle is greater than a second reference plane ordatum in accordance with the present invention;

FIG. 10 is similar to FIG. 9 but for when the flight path angle of theaircraft is less than the second reference plane or datum;

FIG. 11 is a graphical illustration of a cut-off angle correctionboundary for a level flight condition in accordance with the presentinvention;

FIG. 12 is similar to FIG. 11 but for a condition when the flight pathangle of the aircraft is greater than a predetermined reference plane ordatum;

FIG. 13 is similar to FIG. 11 but for a condition when the flight pathangle of the aircraft is less than a predetermined reference plane ordatum and also illustrates a BETA sink rate enhancement boundary inaccordance with the present invention;

FIG. 14 is a graphical illustration of the terrain advisory andsuperimposed terrain warning signals for an aircraft relative to theterrain for a condition when the aircraft flight path angle is less thana first reference plane or datum;

FIG. 15 is similar to FIG. 14 but for a condition when the flight pathangle is between a first datum and a second datum;

FIG. 16 is similar to FIG. 14 but for a condition when the flight pathangle is less than the second datum;

FIG. 17 is a graphical illustration of LOOK-AHEAD/LOOK-DOWN terrainadvisory and warning boundaries in accordance with the present inventionfor a condition when the aircraft is descending;

FIG. 18 is similar to FIG. 17 but for a condition when an aircraft isclimbing;

FIG. 19 is a graphical illustration of LOOK-UP terrain advisory andwarning boundaries in accordance with the present invention;

FIG. 20 is a graphical illustration of an aircraft during a pull-upmaneuver;

FIG. 21 is an exemplary block diagram for a system for generating asignal representative of the altitude loss due to pilot reaction timeALPT, as well as altitude loss due to a pull-up maneuver ALPU inaccordance with the present invention;

FIG. 22 is a graphical illustration of an alternative methodology forgenerating a cut-off altitude boundary in accordance with the presentinvention;

FIG. 23 is a block diagram of the display system in accordance with thepresent invention;

FIG. 24 is a functional diagram of the display data format in accordancewith the ARINC 708/453 standard for a weather radar display;

FIG. 25 is a plan view of a background terrain display illustrating thevariable density dot pattern in accordance with the present invention;

FIG. 26 is similar to FIG. 25 but additionally illustrates the terrainthreat indications;

FIG. 27 is a block diagram illustrating the configuration of a flash ROMfor a terrain data base and a RAM used with the present invention;

FIG. 28 illustrates a wedding-cake configuration for a RAM utilized inthe present invention;

FIG. 29 is a diagram of the file format of the files in the terrain database in accordance with the present invention;

FIG. 30 illustrates a terrain map of the terrain data base in accordancewith the present invention illustrating the highest elevation in each ofthe cells within the map;

FIG. 31 is similar to FIG. 30 and illustrates a method for correctingvarious cells in the vicinity of a runway;

FIG. 32 is a simplified elevational view of certain data illustrated inFIG. 31;

FIG. 33 is a diagram of a display illustrating the weather radar sweepspokes and the determination of the range increments;

FIG. 34 is a block diagram of the display system in accordance with thepresent invention illustrating the memory configuration and the dataflow;

FIG. 34A is a block diagram of a configuration alternative to that shownin FIG. 34;

FIG. 35 is a diagram of a terrain map illustrating the determination ofthe look-ahead vector array in accordance with the present invention;

FIG. 36 is a diagram of a display illustrating a radar sweep spoke and amethod for determining the range increments of the radar sweep spoke;

FIG. 37 is a terrain map illustrating the computation of the hazardousterrain in accordance with the present invention;

FIG. 38 is a diagram of a terrain map superimposed on a display screenillustrating the sweep range and the determination of the incrementalrange increments;

FIG. 39 is a diagram of a pixel map of the display in accordance withthe present invention;

FIG. 40 is a diagram of the different fractal patterns used to providethe variable density display of non-hazardous display indications inaccordance with the present invention;

FIG. 41 is a diagram illustrating the determination of the X and Yincrements of the display screen; and

FIG. 42 is a diagram of the method for superimposing the fractalpatterns on the displayed pixel map in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a terrain awareness system (TAS), generallyidentified with the reference number 20, is illustrated. As will bediscussed in more detail below, the TAS 20 utilizes inputs from asatellite-based navigational system, such as global positioning system(GPS) 22 (longitude, latitude, altitude, groundtrack, ground speed),and/or an FMS/IRS navigational system which may be updated by the GPSand/or DME/DME, a terrain data base 24, an airport data base 26 andcorrected barometric altitude, for example from a barometric altimeter28 to provide a LOOK-AHEAD warning system which provides relativelylonger warning times while minimizing nuisance warnings. Since the TAS20 does not require a radio altimeter input, the system can be used withcertain aircraft, such as commuter aircraft, which are not normallyequipped with a radio altimeter.

The current longitude and latitude of the aircraft from the GPS 22 areapplied to an Airport and Terrain Search Algorithm, indicated by a block29, which includes location search logic for determining the terraindata, as well as the airport data surrounding the aircraft. Such searchlogic is described in detail in U.S. Pat. Nos. 4,675,823 and 4,914,436assigned to the same assignee as the present invention and herebyincorporated by reference. The GPS inputs, along with terrain andairport data surrounding the aircraft from the search algorithm,indicated by the block 29, are applied to the LOOK-AHEAD warninggenerator 30, which provides both terrain advisory and terrain warningsignals based upon the position and projected flight path of theaircraft. The LOOK-AHEAD warning generator 30 may provide both an auralwarning by way of a voice warning generator 32 and speaker 34, and/or avisual warning by way of a map or a display 36, as discussed below.

GLOBAL POSITIONING SYSTEM

The primary positional information for the aircraft is provided by theGPS 22, such as disclosed in U.S. Pat. Nos. 4,894,655; 4,903,212;4,912,645; 4,954,959; 5,155,688; 5,257,195; 5,265,025; 5,293,163;5,293,318; and 5,337,242, all hereby incorporated by reference. The GPS22 includes a GPS receiver 37 and a combination GPS receiver and monitor38. The positional inputs from the GPS 22 are processed by a deadreckoning algorithm, indicated by the block 40, for example as discussedin detail in U.S. Pat. No. 5,257,195, to compensate for temporaryoutages of the GPS 22, such as loss or masking of satellites. The deadreckoning algorithm can be replaced by an additional navigationalsystem, such as an FMS or an IRS, which can be used during GPS outages.

A differential GPS receiver 42 can also be used to provide an indicationof a GPS altitude. As long as the differential GPS information isreceived and four more satellites are visible, the differential GPSinformation from the differential GPS receiver 42 is sufficientlyaccurate for the TAS 20. If the differential GPS receiver 42 is notavailable, a compound altitude signal may be generated by the block 44and connected to the LOOK-AHEAD warning generator 30 by way of asingle-pole, double-throw switch 46, under the control of a signal "DIFFGPS AVAILABLE", which indicates when the differential GPS receiver 42 isavailable. In a first mode when differential GPS information isavailable, the switch 46 connects the GPS altitude signal from the GPS22 to the LOOK-AHEAD warning generator 30. In this position, the GPSaltitude signal is generated either from the GPS 22 or from thedifferential GPS receiver 42, as discussed above. When a differentialGPS receiver 42 is not available, a compound altitude signal is providedby the block 44 to the LOOK-AHEAD warning generator 30. During thiscondition, the switch 46, under the control of a "DIFF GPS AVAILABLE"signal, connects the block 44 to the LOOK-AHEAD warning generator 30 toenable the compound altitude signal to be provided to the LOOK-AHEADwarning generator 30. Within the Differential GPC Receiver 42, thebarometric altitude is compared with the GPS altitude to generate acompound altitude signal which could be used to limit the maximumdifference between the GPS output altitude signal and the barometricaltitude to the maximum expected GPS altitude error. Such aconfiguration would reduce the effect of erroneous pressure correctionsettings on the barometric altimeter 28.

TERRAIN AND AIRPORT DATA BASES

As mentioned above, the airport data and terrain data are separated intotwo different data bases 26 and 24, respectively. Such a configurationallows either of the data bases 24 or 26 to be updated without the needto update the other.

The airport data base 26 contains various types of information relatingto airports, such as runway midpoint coordinates, runway length, runwayheading, runway elevation and airport/runway designation data.Additional data relating to obstacles along the approach path (i.e.high-rise hotels adjacent Heathrow Airport in England) and a nominalfinal approach slope could also be included in the airport data base 26.A separate obstacle data base may also be provided, structured similarto the airport data base.

In order for the TAS 20 to be useful, all airports, for example, whichcommuter planes without radio altimeters can land, would have to beincluded in the airport data base 26. In areas where the airport database 26 is not complete, the search algorithm 29 would indicate NO DATA.Since the airport data base 26 is organized in a similar fashion as theterrain data base 24 as described below, additional airport data couldbe added in a piecewise fashion when available. The estimated size ofthe airport data base 26 is contemplated to be relatively small,compared to the terrain data base 24, for example less than 200kilobytes depending on the amount of data stored per airport.

The terrain data base 24 may require up to 40 megabytes of storagespace, which makes it compatible with available flash erasableprogrammable read only memory (ROM) devices, such as a flash EEPROM, ormini hard disk drives, which could take advantage of known datacompression techniques to reduce the amount of storage space requiredand also promote easy data retrieval by the search algorithm 29.

An important aspect of the invention is that the terrain data base 24 isstructured to, provide varying resolutions of terrain data as a functionof the topography of the terrain, as well as distance to airports. Forexample, a relatively high resolution can be provided close to theairport on the order of 1/4 to 1/8 nautical miles and a mediumresolution, for example 1/2 to 1 nautical miles within a 30-mile radiusof the airport. Outside of the 30-mile radius from the airport, acoarser resolution is sufficient as will be described below.

FIGS. 2 through 4 illustrate the organization of the terrain data base.Referring first to FIG. 2, the world is divided into a plurality oflatitude bands 50, for example each about 4° wide. Each latitude band 50is then divided into a plurality of longitudinal segments 52, which areabout 4° wide around the equator, such that each longitudinal segment 52is about 256×256 nautical miles. In order to maintain a relativelyconstant segment size, the number of longitudinal segments 52 perlatitude band 50 are reduced closer to the poles.

Since the number of latitude bands 50 and the number of longitudinalsegments 52 per latitude band 50 is fixed, determination of theparticular segment corresponding to the current aircraft position isreadily determined. For example, the aircraft current latitude X is usedto determine the latitude band number. Next, the number of longitudinalsegments 52 associated with that band 50 is determined either by way ofa LOOK-UP table or by calculation. Once the number of longitudinalsegments 52 in the particular latitude band 50 of interest isdetermined, the current longitudinal position Y of the aircraft can beused to readily determine the longitudinal segment number.

The minimum data associated with each longitudinal segment 52corresponds to the highest altitude within that segment. As mentionedabove, each of the segments 52 is approximately 256×256 nautical miles.As shown in FIG. 3, these longitudinal segments 52 can be broken downinto various subsquares to provide varying levels of resolution. Forexample, each segment 52 may be broken down into a plurality ofsubsquares 54, each subsquare 54 being 64×64 nautical miles to provide avery coarse resolution. The subsquares 54, can, in turn, be furthersubdivided into a number of subsquares 56, for example, each 16×16nautical miles, to provide a coarse resolution. These subsquares 56, inturn, can be subdivided into a plurality of subsquares 58, for example,each 4×4 nautical miles, to provide a medium resolution. The subsquares58 can also be broken down into a plurality of subsquares 60, forexample 1×1 nautical miles, to provide a fine resolution. In thevicinity around airports, it may even be desirable to break down thesubsquares 60 down into smaller subsquares 62 to provide even finerresolution, for example 1/4×1/4 nautical miles.

As shown in FIG. 4A, the minimum data associated with each longitudinalsegment 52 consists of a header 65 which includes a multiple data byte66 which includes the reference altitude which corresponds to thehighest altitude for all the subsquares within the segment, whichassumes that the highest elevation in a square is representative of theentire square elevation. The multiple data byte 66 may also include aflag to indicate when no further subdividing is required for certaingeographical areas. For example, for segments representing the ocean,all subsquares would have the same maximum altitude and thus no furthersubdividing would be required. In order to enable the data base to becreated and updated on a piecewise basis, the multiple data byte 66 mayalso contain a flag bit or code indicating that data corresponding tofurther subdivisions does not exist for a particular segment 52containing a code indicating that no map data exists in the segment.

For geographical areas, such as mountainous areas and areas in thevicinity of an airport, the longitudinal segments 52 are subdivided asdiscussed above. In such a situation, the stored data would include a2-byte mask word 68, which points to the subsquares having differentaltitudes. For example, as shown in FIG. 4, the 2-byte mask word 68 isillustrated with a pointer in the box 15 within the subsquare 54 withinthe longitudinal segment 52, which represents a different altitude whichpoints to the next finer layer of resolution as discussed above. Thesubsquares each include a header 70 including a mask as described aboveto enable a pointer for each subsquare having a different altitude topoint to the next finer layer until the finest resolution level desireis reached as shown.

The data base structure provides for flexibility as to resolutionrequired and can be constructed on a piecewise basis at different times.As such, the structure contains the entire framework necessary to addnew data or modify existing data without changing any of the software.

The size of the data base is proportional to the resolution and the sizeof the area to be covered. In order to cover the entire earth's surface,about 149 million square nautical miles, only about 2,500 to 3,500longitudinal segments 52 are required. Thus, the overhead for theminimum header sizes is relatively small.

Alternatively, the world may be divided into a plurality of, forexample, 1°×1° map files or cells 51 as shown in FIG. 4B. The variousmap files 51 may be formed with variable resolution. More particularly,each of the map files may be subdivided into a plurality of subcells 53with the highest terrain elevation in each subcell indicated, with theresolution of all of the subcells within a particular map file being ofthe same resolution. As indicated above, the maps are generally 1°×1°,which may be combined for coarser resolution (i.e. 2°×2° or 10°×10°).

The size of the subcells 53 is varied to provide varying degrees ofresolution. For example, for relatively high resolution, the size of thesubcells 53 may be selected as either 15×15 arc seconds or 30×30 arcseconds. The size of the subcells 53 may also be varied as a function ofthe terrain in particular geographic areas. For example, the size of thesubcells 53 may be selected as 30×60 arc seconds at relatively far northlongitudes and as 30×120 arc seconds at longitudes even further north.The size of the subcells may also be varied as a function of thedistance to the nearest airport. For example, for distances greater than64 miles from an airport, the size of the subcells may be selected as60×60 arc seconds while at distances greater than, for example, 128miles from the airport, size of the subcell 53 may be selected as120×120 arc seconds. For a course resolution, for example for use duringa cruise mode as discussed below, the size of subcells may be selectedas 5×5 arc minutes.

As discussed below and as illustrated in FIG. 27, the data base may becompressed by a compression algorithm and stored in a flash read onlymemory (ROM), configured as a wedding cake as illustrated in FIG. 28.Various compression algorithms are known and suitable for thisapplication. The compression algorithm does not form a part of thepresent invention.

In order to simplify computation, each of the map files includes aheader 55, which includes, inter alia, the resolution of the particularmap file 51 as well as the location of one or more corners, for example,the northwest and southeast corners. The headers 55 as well as the mapfiles 51 are discussed in more detail below and illustrated in moredetail in FIG. 29.

For certain terrain, for example, terrain adjacent a runway, theresolution of the map files 51 (i.e. relatively large size of thesubcells 53) may result in errors in runway elevation. Moreparticularly, since each subcell 53 contains the highest elevationwithin the subcell 53, certain errors relating to runway elevations canoccur, depending on the resolution of the map file 51. Correction of theelevations adjacent the runways is discussed in more detail below inconjunction with FIGS. 30-33. Systematic errors may also be introducedby the maps, which are digitized, for example, by the Defense MappingAgency, using known averaging algorithms, which are known to fill invalleys and cut-off the tops of sharp peaks of the terrain.

LOOK-AHEAD WARNING GENERATOR

As mentioned above, the LOOK-AHEAD warning generator 30 generates both aterrain advisory signal and a terrain warning signal based upon theposition and trajectory of the aircraft relative to stored terrain data.There are two aspects of the terrain advisory and terrain warningsignals: LOOK-AHEAD distance/direction; and terrain threat boundaries.

LOOK-AHEAD DISTANCE/DIRECTION

The LOOK-AHEAD direction for detecting threatening terrain is along thegroundtrack of the aircraft. In order to prevent nuisance warnings, theLOOK-AHEAD distance is limited, as will be discussed below. Otherwise,potentially threatening terrain along the current flight path of theaircraft relatively far from its current position could produce nuisancewarnings.

Two different LOOK-AHEAD distances (LAD) are utilized. The first LAD isused for a terrain advisory signal (also referred to as a yellow alertLAD). A second LAD is used for terrain warning signals which requireimmediate evasive action (also referred to as a red-alert LAD).

The LAD for a terrain advisory condition is considered first indetermining the LAD because it is assumed that the pilot could make a30° bank turn at any time at a turning radius R. The LAD is equal to theproduct of the speed of aircraft and the total LOOK-AHEAD time. As shownin FIG. 5, the total LOOK-AHEAD time is equal to the sum of theLOOK-AHEAD time T₁ for a single turning radius R; the LOOK-AHEAD time T₂for terrain clearance at the top of the turn (i.e. point 73) plus apredetermined reaction T₃.

The terrain clearance at the top 73 of the turn is provided to preventinadvertent terrain contact as a result of the turn. This terrainclearance may be selected as a fixed distance or may be made equal tothe turning radius R.

As shown in equation (1), the turning radius R is proportional to thesquare of the speed of the aircraft and inversely proportional to thebank angle TG (ROLL).

    R=V.sup.2 /G×TG (Roll)                               (1)

For a 30° bank angle, the turning radius R in nautical miles (nM) as afunction of the speed in knots is given by equation (2).

    R=0.000025284*V.sup.2                                      (2)

The LOOK-AHEAD time T₁ for a single turning radius is given by equation(3).

    T.sub.1 =R/V                                               (3)

Substituting R from equation (1) into equation (3) yields the LOOK-AHEADtime T₁ for a single turn radius as shown in equation (4) as a functionof the speed of the aircraft and the bank angle.

    T=V/G×TG (Roll)                                      (4)

TABLE 1 provides various LOOK-AHEAD times T₁ at various turning radiiand ground speeds.

                  TABLE 1                                                         ______________________________________                                        Radius          Speed   T.sub.1                                               (nM)            (knots) (sec)                                                 ______________________________________                                        1/2             140     13                                                    1               200     18                                                    2               280     26                                                    3               345     31                                                    4               400     38                                                    5               445     40                                                    6               490     44                                                    ______________________________________                                    

By assuming that the fixed terrain clearance at the top 73 of the turnis equal to one turning radius R, the total LOOK-AHEAD time for two turnradii (i.e. T₁ +T₂) is simply twice the time for a single turningradius. Thus, the total LOOK-AHEAD time is 2*T₁ plus the predeterminedreaction time T₃, for example 10 seconds, as given by equation (5).

    T.sub.TOTAL =2*T.sub.1 +T.sub.3                            (5)

By substituting R from equation (2) into equation (4), the LOOK-AHEADtime for a single turn radius is given by equation (6).

    T.sub.1 =0.000025284*V                                     (6)

Thus, if it is assumed that the fixed clearance X at the top of the turn73 is also equal to the turning radius R, the total LOOK-AHEAD time fortwo turning radii is given by equation (7) below, simply twice the valuedetermined in equation (6).

    T.sub.1 +T.sub.2 =2*0.000025284*V                          (7)

For a predetermined reaction time T₃, for example 10 seconds, theLOOK-AHEAD distance (LAD) in nautical miles for a terrain advisorysignal can be determined simply by multiplying the speed of the aircraft(V) by the total time T_(TOTAL) as shown in equation (8).

    LAD=V*(0.0000278+2*0.000252854*V)+fixed distance*K,        (8)

where K is a constant, for example 0.

However, in order to provide optimal conditions during a terrainadvisory condition, the LAD is limited with an upper limit and a lowerlimit. The lower limit may be a configurable amount, for example, either0.75, 1 or 1-1/2 nautical miles at relatively low speeds (i.e. speedsless than 150 knots) and limited to, for example, 4 nautical miles atspeeds, for example, greater than 250 knots. The LAD may also be limitedto a fixed amount regardless of the speed when the distance to therunway is less than a predetermined amount, for example 2 nauticalmiles, except when the aircraft altitude is greater than 3,000 feet,relative to the runway.

The terrain warning LAD is given by equation (9) below.

    LAD=k.sub.1 *LAD (terrain for LOOK-DOWN/LOOK-AHEAD advisory indication),(9)

    k.sub.2 *LAD (terrain LOOK-DOWN/LOOK-AHEAD warning),

    k.sub.3 *LAD (terrain LOOK-UP advisory),

where k₁ =1.5, except when the LAD is limited at its lower limit, inwhich case k₁ =1, k₃ =1 and where k₂ =0.5, k₃ =2.

TERRAIN THREAT BOUNDARIES

The terrain threat boundaries include a terrain floor boundary, terrainadvisory boundaries (yellow alert), as well as terrain warningboundaries (red alert) and are provided along the groundtrack of theaircraft.

TERRAIN FLOOR BOUNDARY

The terrain floor boundary is the basis for the terrain threatboundaries and is similar to the terrain floor developed for the GPWS.The terrain floor relates to a distance ΔH below the aircraft and isproportional to the distance to the closest runway to prevent nuisancewarnings when the aircraft is taking off and landing, while providingadequate protection in other modes of operation. As illustrated inequation (10), the terrain floor boundary below the aircraft isessentially based upon providing 100 feet clearance per nautical milefrom the runway, limited to 800 feet.

    ΔH=100 (ft/nM)*(distance to runway threshold-an offset)+100 (ft/nM)*(distance to runway center-12 nautical miles)     (10)

Equation (10) is shown graphically in FIG. 6. Referring to FIG. 6, thehorizontal axis represents the distance from the runway, while thevertical axis represents the ΔH terrain floor boundary beneath theaircraft. The first segment 71 represents the runway length, while thepoint 72 represents the runway center. After a small offset 74, the nextsegment 78, which starts at 0 feet, slopes at 100 feet per nautical miledistance from the runway threshold, identified by the point 80, up to amaximum of 500 feet. The 500 foot maximum continues along a segment 82until the distance to the runway center 72 is a predetermined distanceD, for example 12 nautical miles. The next segment 84 slopes up at a 100feet per nautical mile distance until the distance rises 300 feet fromthe segment 82 for a total of 800 feet.

The terrain floor ΔH boundary beneath the aircraft is limited such thatthe segment 78 begins at 0 and the segment 82 never goes above apredetermined maximum, for example 500 feet. In addition, the terrainfloor ΔH boundary is also limited so that the segment 84 never goesbelow the vertical height of the segment 82 and rises no more than 300feet relative to the segment 82. During conditions when the aircraft is3,000 feet above the terrain, the terrain floor ΔH boundary ismaintained at the upper limit of 800 feet.

TERRAIN ADVISORY BOUNDARIES

Two terrain advisory boundaries are shown graphically in FIGS. 7 and 8.FIG. 7 represents a condition when the aircraft is descending while FIG.8 represents a condition when the aircraft is ascending. As will bediscussed in more detail below, the terrain advisory boundaries arebased upon the relationship between the flight path angle γ and a firstconfigurable datum, THETA1.

Referring first to FIG. 7, the terrain advisory (yellow alert)boundaries are shown. The first segment of the terrain advisoryboundary, identified with the reference numeral 92, corresponds to theΔH terrain floor boundary. As mentioned above, the ΔH terrain floorboundary is a function of the distance from the runway 94. In order todetermine the bottom segment 100 of the terrain advisory boundary, it isnecessary to determine whether the flight path angle γ is more or lessthan a configurable datum, identified as THETA1. As shown and describedherein, THETA1 is set for 0°. However, it should be clear that otherangles for THETA1 are within the broad scope of the present invention.For the condition illustrated in FIG. 7, the flight path angle γ is lessthan THETA1. Thus, the segment 100 extends from the ΔH terrain floorboundary at the angle THETA1 to the LOOK-AHEAD distance for a terrainadvisory. The final segment 102 is generally parallel to the segment 92and extends in a vertical direction along the LAD for the terrainadvisory boundary.

If the flight path angle γ is greater than THETA1 as shown in FIG. 8.,then different terrain advisory boundaries are provided. The segment 92will be the same as shown in FIG. 7 and described above. However, whenthe flight path angle γ is greater than THETA1 as shown in FIG. 8, abottom segment 106 is formed by extending a line segment from the bottomof the segment 92, which represents the ΔH terrain floor, along adirection parallel with the flight path angle γ up to the LAD. Avertical segment 108 extends from the bottom segment 106 along the LADto form the terrain advisory boundaries.

Thus, as shown, if the airplane is climbing, only terrain whichpenetrates the upward-sloping terrain advisory boundaries will cause anadvisory signal, such that if the aircraft clears the terrain by a safemargin, no advisory signal is given, even if the terrain ahead is at orabove the present aircraft altitude. However, if terrain penetrates theterrain advisory boundaries, an aural warning, such as "CAUTION,TERRAIN" aural advisory may be given. In addition, visual warnings maybe provided.

TERRAIN WARNING BOUNDARIES

The terrain warning boundaries are shown in FIGS. 9 and 10 and, ingeneral, indicate to the pilot of an aircraft conditions when evasiveaction is required to avoid terrain contact. The terrain warningboundaries are based on the relationship of the flight path angle of theaircraft relative to a second configurable datum, THETA2. The datumTHETA2 may be selected with an upslope of, for example, 6°, which isequal to the average climb capability of airliners. The datum THETA2could be modified, taking into consideration aircraft type,configuration, altitude and time for takeoff. However, because of thelonger LOOK-AHEAD distance as discussed above for the terrain warningboundaries, a terrain warning could occur before a terrain advisoryindication for extreme terrain conditions.

Referring to FIGS. 9 and 10, two different terrain warning boundariesare illustrated. In particular, FIG. 9 illustrates a condition when theflight path angle γ is greater than the second datum THETA2. FIG. 10illustrates a condition when the flight path angle γ is less than thesecond datum THETA2. In both conditions, the first segment 114 of theterrain warning boundary is extended below the aircraft for a distance1/2 of the ΔH terrain floor, which, as mentioned above, is a function ofthe distance of the aircraft from a runway. As mentioned above, theterrain warning boundaries are dependent on the relationship between theflight path angle γ of the aircraft and the datum THETA2.

Referring first to FIG. 9, since the flight path γ is less than thesecond datum THETA2, the bottom segment 118 of the terrain warningboundary is formed by extending a line from the segment 114 at an angleequal to the flight path angle γ up to the LOOK-AHEAD distance for aterrain warning as discussed above. A vertical segment 120 extends fromthe lower segment 118 along the LAD for a terrain warning, formingthe-terrain warning boundaries illustrated in FIG. 9.

If the flight path angle of the aircraft is less than the slope of thesecond datum THETA2 as shown in FIG. 10, a bottom segment 122 extendsfrom the segment 114 up to the LOOK-AHEAD distance at an angle equal tothe flight path angle γ. A segment 124 extends upwardly along the LADfrom the segment 122. If any terrain along the groundtrack of theaircraft out to the LAD for the terrain warning penetrates the envelope,aural and/or visual terrain ahead warnings may be given.

CUT-OFF BOUNDARIES

In order to avoid spurious warnings when the aircraft overflies a ridgeat relatively low altitudes, the warning boundaries may include cut-offboundaries, for example, as illustrated in FIGS. 11, 12 and 13. Withoutthe cut-off boundaries, warnings would be given, although the terrain isvirtually below the aircraft and no terrain is visible ahead. Referringfirst to FIG. 11, the cut-off boundary 126 begins at a predeterminedcut-off offset 128 below the aircraft and extends in a direction infront of the aircraft at a predetermined envelope cut-off angle 130. Theenvelope cut-off angle 130 is equal to the flight path angle γ plus aconfigurable predetermined cut-off angle, described and illustrated as-6°. For level flight as shown in FIG. 11, the cut-off boundary 126extends from the cut-off offset 128 in the direction of the envelopecut-off angle 130 toward the front of the aircraft to a point 132 whereit intersects a terrain advisory boundary or terrain warning boundary,identified with the reference numeral 134. For level flight, as shown inFIG. 11, the flight path angle γ is zero. Thus, the cut-off boundary 126illustrated in FIG. 11 will extend from the cut-off offset 128 along anangle equal to the cut-off angle, which, as mentioned above, is selectedas -6° for illustration. As mentioned above, the cut-off boundary 126extends from the cut-off offset 128 to the point 132 where it intersectsthe terrain advisory boundary 134 discussed above. The warning boundaryis then selected to be the highest of the terrain advisory boundary 134and the envelope cut-off boundary 126. Thus, for the example illustratedin FIG. 11, the terrain advisory boundary would consist of the cut-offboundary 126 up to the point 132, where the envelope cut-off boundary124 intersects the warning envelope 126. From the point 132 forward, thenormal terrain advisory boundary 134, corresponding, for example, to aTHETA1 slope, is utilized. Thus, if either a terrain advisory boundaryor terrain warning boundary is below the cut-off boundary 126, thecut-off boundary 126 becomes the new boundary for the advisory orwarning signal.

The cut-off angle 130 is limited such that if the flight path angle γ isa relatively high climb angle, the cut-off angle 130 does not exceed apredetermined limit, for example a configurable limit between 0 to 6° aswill be illustrated in FIG. 12. In particular, referring to FIG. 12, theflight path angle γ is shown at a relatively high climb rate (i.e.γ=9°). An unlimited cut-off envelope angle 130 is illustrated by thepartial segment 136. As shown, this segment 132 is 6° below the flightpath angle γ However, in order to decrease the sensitivity of the system20 during conditions when the flight path angle is relatively large, forexample 9°, indicative of a fairly steep climb, the cut-off envelopeangle 130 is limited. In the example illustrated in FIG. 12, the cut-offangle limit is selected, for example, to be 1.4°. Thus, at flight pathangles γ greater than 7.4°, the cut-off angle will result in an envelopecut-off boundary 140, which is more than 6° below the flight path angleγ. As shown, since the cut-off angle limit was selected as 1.4°, thecut-off angle will be limited to 1.4° anytime the flight path angle isgreater than 7.4°, even though the cut-off angle will be equal to ormore than 6° less than the flight path angle γ. Thus, for the conditionsillustrated in FIG. 12, the segment 140 begins at the cut-off offset 128and continues along a line that is 1.4° from the horizon until itintersects the terrain advisory or terrain warning boundary 142 at thepoint 144. The actual advisory or warning boundary will be formed by thelimited cut-off boundary 140 up to the point 144 and will continue withthe normal warning boundary 142 beyond the point 144.

FIG. 13 illustrates a situation-when the aircraft is descending and thusthe flight path angle γ is less than 0. During conditions when theflight path angle γ is less than the normal descent angle, the warningboundary is lowered past the cut-off boundary for high sink-ratesituations in order to prevent nuisance warnings during step-downapproaches. In particular, a BETA sink rate enhancement is provided andis effective anytime the flight path γ is below a predeterminedconfigurable descent bias GBBIAS, for example, -4°. During suchconditions, an angle BETA is added to the terrain advisory datum(THETA1) as indicated below. In particular, when the flight path angle γis greater than the descent bias angle GBBIAS or the flight path angle γis less than zero, BETA is selected as 0. When the flight path angle γis greater than zero, BETA is given by equation (11).

    BETA=γ-THETA1; for γ>0                         (11)

When the flight path angle γ is less than GBBIAS, the value for BETA isselected in accordance with equation (12) below.

    BETA=K*(γ-GBBIAS),                                   (12)

where k=0.5, γ=flight path angle and GBBIAS is selected as -4°.

FIG. 13 illustrates a condition when the flight path angle γ is -8°.During this condition, the cut-off angle 130 is selected to be 6° belowthe flight path angle γ and thus defines an envelope cut-off boundary150 that extends from the cut-off offset 128 along a line which is -14°from the horizontal axis. Since the flight path angle γ is less thanGBBIAS, the angle BETA is added to the normal warning boundary accordingto equation (12) above. Thus, for a flight path angle of -8°, the BETAangle will be 2°. Thus, the terrain advisory or warning boundary,indicated by the boundary 151, instead of extending along the THETA1slope, will extend along an angle which is 2° below the THETA1 slope todefine the enhanced boundary 152. The warning cut-off boundary 152 thuswill extend to the point 156 where it intersects the normal terrainadvisory or terrain warning boundary 152. Since the cut-off angleboundary 150 is higher than boundary 152 up to the point 156, thecut-off boundary 150 will be utilized as the warning boundary up to thepoint 156. Beyond point 156, the BETA-enhanced warning boundary 152 willdefine the warning envelope.

FIGS. 14, 15 and 16 illustrate the composite terrain advisory andterrain warning boundaries for different conditions. The cross-hatchedarea illustrates terrain in the vicinity of the aircraft. Threeconditions are illustrated. In particular, FIG. 14 represents acondition when the flight path angle γ of the aircraft is less than thefirst datum THETA1. FIG. 15 represents a condition when the flight pathangle γ is between the first datum THETA1 and the second datum THETA2.FIG. 16 represents a condition when the flight path angle γ is greaterthan the second datum THETA2. The terrain advisory boundaries areindicated in solid line while the terrain warning boundaries areillustrated by dashed lines.

Referring first to FIG. 14, the terrain advisory boundaries include aboundary 155, which extends from the aircraft to the ΔH terrain floor.Normally, the lower terrain advisory boundary 157 would extend from thesegment 155 along an angle equal to the THETA1 slope. However, in thissituation, the flight path angle γ is illustrated to be greater than theGBBIAS angle; therefore, a BETA-enhanced boundary 159, as discussedabove, is utilized which extends along an angle that is equal to theTHETA1 angle plus the BETA angle, up to the LOOK-AHEAD distance for aterrain advisory. A vertical boundary 158 extends from the first datumTHETA1 vertically upwardly along the LOOK-AHEAD distance for a terrainadvisory. Since the cut-off boundary 157 is higher than the terrainadvisory boundary 157 up to the point 169 where the two boundariesintersect, the cut-off boundary 157 becomes the terrain advisoryboundary up to the point 169. Beyond the point 169, the boundary 159 isutilized as the warning boundary.

The terrain warning boundary includes a boundary 162 extending to apoint equal to 1/2 of the terrain floor as discussed above. A boundary165 extends from the boundary 162 at the THETA2 slope. A verticalboundary 167 extends from the boundary 165 along the LOOK-AHEAD distancefor a terrain warning as discussed above.

FIG. 15 illustrates a situation when the flight path angle γ is greaterthan the THETA1 angle and less than or equal to the THETA2 angle. Duringthis condition, the boundary for the terrain warning extends along asegment 180, which is at 1/2 of the ΔH terrain floor. A sloping boundary184 extends from the segment 180 to the LOOK-AHEAD distance for aterrain warning (point 186). From there, a vertical boundary 188 extendsvertically upwardly to define the terrain warning boundaries.

The terrain advisory boundary includes a boundary 185 which extendsdownwardly to the terrain floor, i.e. point 187. From point 187, theterrain advisory boundary extends along a segment 189 at the flight pathangle γ up to a point 191, the LOOK-AHEAD distance for a terrainadvisory. A vertical segment 194 extends along the LAD from the boundary189 at the point 191.

In this illustration, an envelope cut-off boundary 193 cuts off aportion of the terrain advisory indication boundary since the flightpath angle γ of the aircraft is greater than a predetermined limit.Thus, the portions of the warning boundary 189 below the cut-off angleboundary 193 are ignored; and the segment 193 becomes the effectiveterrain advisory indication up to the point 195, where the cut-offboundary 193 intersects the terrain advisory indication boundary 189.After the point 195, the boundary 189 will be the effective warningboundary.

FIG. 16 illustrates a situation when the flight path angle γ is greaterthan THETA2. In this situation, a terrain advisory boundary 200 extendsfrom the aircraft to the boundary terrain floor. A lower boundary 202extends at the flight path angle γ from the boundary 200 up to theLOOK-AHEAD distance for a terrain advisory. A vertical terrain advisoryboundary 204 extends upwardly along the LOOK-AHEAD distance for theterrain advisory. Since the flight path angle γ is illustrated to thegreater than GBBIAS, a segment 211, the cut-off boundary, defines theeffective warning boundary up to a point 213 where the terrain advisoryboundary intersects the cut-off warning boundary. After that point,segment 202, which is higher, becomes the warning boundary.

The terrain warning boundary extends along a segment 208 down to a pointof 1/2 the terrain floor. A lower boundary 210 extends along an angleequal to flight path angle γ since it is greater than the THETA2 slope.A vertical terrain warning boundary 212 extends upwardly from theboundary 210 along the LOOK-AHEAD distance for a terrain warning.

In order to provide additional sensitivity of the warning system,terrain advisory signals within a predetermined portion of the LAD for aterrain advisory, for example 1/2 LAD, may be treated as terrainwarnings. Thus, as shown in FIGS. 14, 15 and 16, vertical boundaries161, 181 and 201, respectively, are shown, for example, at 1/2 LAD.Based upon the flight path angles shown, these segments define effectiveterrain advisory envelopes 166, 182 and 203.

ALTERNATIVE TERRAIN THREAT BOUNDARIES

Alternative terrain threat boundaries are illustrated in FIGS. 17-22.Similar to the terrain threat boundary discussed above and illustratedin FIGS. 5-16, the alternative terrain threat boundaries include aterrain floor boundary, terrain advisory boundaries (yellow alert) andterrain warning boundaries (red alert). In the alternative embodiment,the advisory and warning boundaries are further divided into two parts,a LOOK-AHEAD/LOOK-DOWN boundary for detecting terrain ahead or below theaircraft (FIGS. 17 and 18) and a LOOK-UP boundary for detectingprecipitous high terrain ahead of the aircraft which may be difficult toclear (FIG. 19).

LOOK-AHEAD/LOOK-DOWN TERRAIN ADVISORY AND WARNING BOUNDARIES

As mentioned above, the LOOK-AHEAD/LOOK-DOWN boundaries are illustratedin FIGS. 17 and 18. Referring first to FIG. 17, LOOK-AHEAD/LOOK-DOWNadvisory and warning boundaries are illustrated for a condition when theaircraft is descending (i.e. γ<0) During such a configuration, the firstsegment of the LOOK-AHEAD/LOOK-DOWN terrain advisory boundary,identified with the reference numeral 300, corresponds to the ΔH terrainfloor boundary. As discussed above, the ΔH terrain floor boundary is afunction of the aircraft from the runway. In order to determine thebottom segment 302 of the LOOK-AHEAD/LOOK-DOWN terrain advisoryboundary, the flight path angle γ is compared with a configurable datum,THETA1, for example 0°. During descent conditions, the flight path angleγ will thus be less than THETA1. Thus, the LOOK-AHEAD/LOOK-DOWN terrainadvisory boundary segment will extend from the ΔH terrain floor boundarysegment 300 along the angle THETA1 to the look-ahead distance for aterrain advisory (LAD). The final segment 304 extends verticallyupwardly from the segment 302 along the LAD.

The LOOK-AHEAD/LOOK-DOWN terrain advisory boundary may also be modifiedby the BETA sink rate enhancement as discussed above. In thisembodiment, the BETA sink rate enhancement ensures that an advisoryindication always precedes a warning indication when the aircraftdescends into or on top of terrain. The BETA sink rate enhancement isdetermined as a function of the flight path angle γ and two (2)configurable constants KBETA and GBIAS. The BETA sink rate enhancementBETA1 for a LOOK-AHEAD/LOOK-DOWN terrain advisory is provided inequation (13) below for a condition when the aircraft is descending.

    BETA1=KBETA*(γ-GBIAS),                               (13)

where GBIAS is a configurable constant, selected for example,. to bezero (0) and KBETA is also a configurable constant selected, forexample, to be 0.5.

Referring to FIG. 17, the BETA sink rate enhancement BETA1 for theLOOK-AHEAD/LOOK-DOWN terrain advisory boundary provides an advisorywarning at a distance less than 1/2 LAD, unlike the terrain advisoryboundary described above). More particularly, for the values definedabove for the configurable constants KBETA and GBIAS, the BETA sink rateenhancement BETA1 results in a segment 306 which extends from the ΔHterrain floor segments 300 at an angle equal to γ/2 up to 1/2 of theLAD. Beyond 1/2 LAD, a segment 308 extends at the angle THETA1 to adistance equal to the LAD. A vertical segment 310 extends along the LADto connect the segments 308 to the segment 304.

As discussed above, a cut-off boundary may be provided to reducenuisance warnings. Referring to FIG. 17, the cut-off boundary isidentified with the reference numeral 312. The cut-off boundary 312extends from a vertical distance 314, below the aircraft along a cut-offangle up to the point of intersection 316 with the terrain advisoryboundary. For distances less than the intersection 316, the cut-offboundary 312 forms the terrain advisory boundary. For distances beyondthe point of intersection 316, the boundaries 306 and 308 form theterrain advisory boundaries.

The terrain warning boundary includes the segment 300 extending from theaircraft along the ΔH terrain floor. A bottom segment 318 connects tothe segment 300 and extends along a BETA sink rate enhancement angleBETA2. The BETA sink rate enhancement angle is determined as a functionof the flight path angle γ and a configurable constant KBETA2 as well asthe constant GBIAS discussed above and provided in equation (14) below.

    BETA2=KBETA2*(GAMMA-GBIAS),                                (14)

where GBIAS is a configurable constant selected, for example, to be 0and KBETA2 is also a configurable constant selected, for example, to be0.25.

For such values of the constants KBETA2 and GBIAS, the BETA enhancementangle KBETA2 will be 1/4*γ. Thus, the segment 318 extends from thesegment 300 at an angle equal to 1/4*γ up to 1/2 the LAD. A verticalsegment 320 extends along a distance equal to 1/2*LAD from the segment318 to define the terrain warning boundary.

Similar to the above, the terrain warning boundaries are also limited bythe cut-off boundary 312. Thus, the cut-off boundary 312 forms theterrain warning boundary up to a point 322, where the cut-off boundary312 intersects the lower terrain warning boundary 318. At distancesbeyond the point of intersection 322, the segment 318 forms the lowerterrain warning boundary up to a distance equal to 1/2 of the LAD.

The terrain advisory and terrain warning boundaries for a condition whenthe aircraft is climbing (i.e. γ>0) is illustrated in FIG. 18. Duringsuch a condition, the BETA sink rate enhancement angles BETA1 and BETA2are set to a configurable constant, for example, zero (0).

The terrain advisory boundary during a climbing condition is formed byextending a vertical segment 324 from the aircraft for a distance belowthe aircraft equal to the ΔH terrain floor. During a climbing condition,a segment 326 is extended from the segment 324 to the LAD at an angleequal to the flight path angle γ. At a point 328 where the segment 326intersects a position equal to 1/2 of the LAD, a vertical segment 330 isextended up from the segment 326, forming a first vertical boundary forthe terrain advisory condition. The line segment 326 from the point 328to the LAD forms the lower terrain advisory boundary while a linesegment 332 extending vertically upward from the line segment 326 alongthe LAD forms a second vertical boundary.

For the exemplary condition illustrated, a cut-off boundary 334 does notintersect the terrain advisory boundaries. Thus, the terrain advisoryboundaries for the exemplary condition illustrated is formed by thesegments 330 and 332 and that portion of the line segment 326 betweenthe line segments 330 and 332.

The terrain warning boundaries for a condition when the aircraft isclimbing includes the vertical segment 324 (FIG. 18) which extends fromthe aircraft to vertical distance equal to the ΔH terrain floor belowthe aircraft forming a first vertical boundary. For a condition when theaircraft is climbing, the line segment 326 extends from the segment 324at the flight path angle γ to form the lower terrain warning boundary.The vertical segment 330 at a distance equal to 1/2 of the LAD forms thesecond vertical terrain warning boundary.

The cut-off boundary 334 limits a portion of the terrain warningboundary. In particular, the cut-off boundary 334 forms the lowerterrain warning boundary up to a point 340, where the cut-off boundary334 intersects the line segment 326. Beyond the point 340, a portion 342of the line segment 340 forms the balance of the lower terrain warningboundary up to a distance equal to 1/2 of the LAD.

LOOK-UP TERRAIN ADVISORY AND WARNING BOUNDARIES

The LOOK-UP terrain advisory and terrain warning boundaries areillustrated in FIG. 19. As will be discussed in more detail below, theLOOK-UP terrain advisory and warning boundaries start at altitudesDHYEL2 and DHRED2, respectively, below the aircraft. These altitudesDHYEL2 and DHRED2 are modulated by the instantaneous sink rate (i.e.vertical speed, HDOT of the aircraft). The amount of modulation is equalto the estimated altitude loss for a pull-up maneuver, for example, at1/4G (i.e. 8 ft/sec²) at the present sink rate. The altitudes DHRED2 andDHYEL2 are dependent upon the altitude loss during a pull-up maneuver(AlPU) and the altitude loss due to reaction time (ALRT) discussedbelow.

The estimated altitude loss due to a pull-up maneuver ALPU is bestunderstood with reference to FIG. 20, wherein the vertical axis relatesto altitude in feet and the horizontal axis relates to time in seconds.The trajectory of the aircraft from a time shortly before pull-up isinitiated to a time when the aircraft has recovered is shown andidentified with the reference numeral 344.

Assuming an advisory or warning indication is generated at time T₁ at apoint 346 along the trajectory 344, the altitude loss due to thereaction time of the pilot (ALRT) is given by equation (15) below.

    ALRT=HDOT*T.sub.R,                                         (15)

where HDOT equals the vertical acceleration of the aircraft in feet/secand T_(R) equals the total reaction time of the pilot in seconds.

Assuming a pull-up maneuver is initiated at time T₂, the altitude lossdue to the pull-up maneuver ALPU may be determined by integrating thevertical velocity HDOT with respect to time as set forth in equation(16) below.

    HDOT(t)=a*t+HDOT.sub.0,                                    (16)

where "a" equals the pull-up acceleration and HDOT₀ is a constant.

Integrating both sides of equation (16) yields the altitude loss as afunction of time H(t) as provided in equation (17) below.

    H(t)=1/2*a*t.sup.2 +HDOT.sub.0 t                           (17)

Assuming a constant acceleration during the pull-up maneuver, the time tuntil vertical speed reaches zero (point 348) is given by equation (18).

    t=-HDOT.sub.0 /a                                           (18)

Substituting equation (18) into equation (17) yields equation (19).

    ALPU=-(HDOT.sub.0).sup.2 /(2*a)                            (19)

Equation (19) thus represents the altitude loss during the pull-upmaneuver.

An exemplary block diagram for generating the signals ALTR and ALPU isillustrated in FIG. 21. In particular, a signal representative of thevertical velocity of the aircraft HDOT, available, for example, from abarometric altimeter rate circuit (not shown), is applied to a filter350 in order to reduce nuisance warnings due to turbulence. The filter350 may be selected with a transfer function of 1/(TAHDOT*S+1); whereTAHDOT is equal to one second. The output of the filter 350 is a signalHDOTf, which represents the filtered instantaneous vertical speed;positive during climbing and negative during descent.

In order to generate the altitude loss due to reaction time signal ALTR,the signal HDOTf is applied to a multiplier 352. Assuming a pilotreaction time Tr, for example, of 5 seconds, a constant 354 equal to 5seconds is applied to another input of the multiplier 352. The output ofthe multiplier 352 represents the signal ALTR, which is positive whenHDOTf is negative and set to zero if the signal HDOTf is positive, whichrepresents a climbing condition. More particularly, the signal HDOTf isapplied to a comparator 356 and compared with a reference value, forexample, zero. If the comparator 356 indicates that the signal HDOTf isnegative, the signal HDOTf is applied to the multiplier 352. Duringclimbing conditions, the signal HDOTf will be positive. During suchconditions, the comparator 356 will apply a zero to the multiplier 352.

The altitude loss due to the pull-up maneuver signal ALPU is developedby a square device 358, a divider 360 and a multiplier 362. The filteredinstantaneous vertical speed signal HDOTf is applied to the squaredevice 358. Assuming a constant acceleration during the pull-upmaneuver, for example, equal to 8 feet/ sec² (0.25 G), a constant isapplied to the multiplier 362 to generate the signal 2a. This signal,2a, is applied to the divider 360 along with the output of the squaredevice 350. The output of the divider 360 is a signal (HDOT f)² /2a,which represents the altitude loss during a pull-up maneuver signalALPU.

These signals ALRT and ALPU are used to modulate the distance below theaircraft where terrain advisory and terrain warning boundaries beginduring a LOOK-UP mode of operation. More particularly, during such amode of operation, ΔH terrain floor segment of the terrain advisoryboundary DHYEL2 is modulated by the signals ALRT and ALPU while the ΔHterrain floor segment of the terrain warning boundary DHRED2 ismodulated by the signal ALPU as indicated in equations (20) and (21),respectively.

    DHYEL2=3/4*ΔH+ALRT+ALPU                              (20)

    DHRED2=1/2*Δ+ALPU,                                   (21)

where ΔH represents the terrain floor as discussed above.

Thus, referring to FIG. 19, the LOOK-UP terrain advisory warning beginsat a point 364 below the aircraft; equal to DHYEL2. If the flight pathangle γ is less than a configurable datum THETA2, a terrain advisoryboundary 366 extends from the point 364 to the advisory LAD at an angleequal to THETA2. Should the flight path angle γ be greater than THETA2,the lower advisory boundary, identified with the reference numeral 368,will extend from the point 364 at an angle equal to the flight pathangle γ.

Similarly, the LOOK-UP terrain warning boundary begins at point 370below the aircraft; equal to DHRED2. If the flight path angle γ is lessthan the angle THETA2, a warning boundary 372 extends from the point 370at angle THETA2 to the warning LAD. Should the flight path angle γ begreater than THETA2, a warning boundary 374 will extend at an angleequal to the flight path angle γ between the point 370 and the warningLAD.

CUT-OFF ALTITUDE

The cut-off altitude is an altitude relative to the nearest runwayelevation; set at, for example, 500 feet. Altitudes below the cut-ofaltitude are not displayed and are ignored by the terrain advisory andterrain warning computations.

The use of a cut-off altitude declutters the display discussed belowaround airports, especially during a final approach when the aircraftapproaches the ground. A second advantage is that nuisance warnings on afinal approach, due to altitude errors, terrain data base resolution andaccuracy errors are minimized.

However, the use of a cut-off altitude during certain conditions, suchas an approach to an airport on a bluff (i.e. Paine Field) at arelatively low altitude or even at an altitude below the airportaltitude, may compromise system performance. More particularly, duringsuch conditions, the use of a cut-off altitude may prevent a terrainwarning from being generated because the threatening terrain may bebelow the cut-off altitude. In order to account of such situations, thesystem selects the lower of two cut-off altitudes; a nearest runwaycut-off altitude (NRCA) and a cut-off altitude relative to aircraft(CARA). The NRCA is a fixed cut-off altitude, relative to the nearestrunway. The CARA is an altitude below the instantaneous aircraftaltitude (ACA) by an amount essentially equivalent to ΔH terrain floorboundary for an advisory condition, which, as discussed above, is afunction of the aircraft distance to the nearest runway.

Equations (22) and (23) below set forth the NRCA and CARA. As mentionedabove, the absolute cut-off altitude (ACOA) is the lower of the NRCA andCARA as set forth in equation (24).

    NRCA=COH+RE,                                               (22)

where COH relates to the cut-off height and is a fixed configurablevalue, initially set between 400 feet and 500 feet; and RE equals therunway elevation.

    CARA=ACA-ΔH-DHO,                                     (23)

where ACA is the instantaneous aircraft altitude; ΔH is the terrainfloor which is proportional to the distance to the runway as discussedabove; and DHO is a configurable bias, set to, for example, 50 feet.

    ACOA=lower of CARA, NRCA,                                  (24)

except where ΔH DH1, in which case ACOA is always equal to NRCA.

DH1 is a point at which the ACOA is forced to be equal to NRCAindependent of the aircraft altitude. The point DH1 is related to COH,ΔH and DHO such that on a nominal three (3) degree approach slope, CARAis equal to NRCA when the aircraft is at a distance equal to a distanceDH1 from the airport, as illustrated in TABLE 2 below.

                  TABLE 2                                                         ______________________________________                                        COH          DH1    DISTANCE TO                                               (feet)       (feet) RUNWAY (n mile)                                           ______________________________________                                        300          50     1                                                         400          100    1.5                                                       500          150    2                                                         ______________________________________                                    

The point DH1 forces the cut-off altitude (COH) above the runwaywhenever the aircraft is close to the runway to ensure robustnessagainst nuisance warnings caused by altitude errors and terrain database resolution margins to disable the terrain advisory and warningindications when the aircraft is within the aircraft perimeter. Thereare trade-offs between nuisance warnings and legitimate warnings. Inparticular, the lower COH, the closer the advisory and warningindications are given, making the system more nuisance prone. Asindicated above, for a COH of 400, terrain advisory and warningindications are effectively disabled when the aircraft is closer than1.5 n miles from the airport runway.

FIG. 22 illustrates the operation of the alternative cut-off altitudeboundaries. In particular, FIG. 22 illustrates a condition when the COHis set to 300 feet with DH1 equal to 50 feet. The cut-off altitudesubstantially for area from the runway, for example, greater than 4nautical miles, is 300 feet, as indicated by the segment 378 when theglide slope angle is less than a predetermined angle, for example, 3°.As the aircraft gets closer to the runway, the COH is lowered, asillustrated by the segment 388, until the aircraft is within one (1)nautical mile of the runway, at which point the COH is forced to be 300feet, which effectively disables any terrain advisory and warningindications within the aircraft is closer than one (1) nautical milefrom the runway, as represented by the segment 382.

During a condition when the aircraft is on a, for example, 3° glideslope angle, the ACOA is forced to be the NRCA. As shown, the NRCA isillustrated by the segment 382.

DISPLAY SYSTEM

A display system, generally identified with the reference numeral 400,is illustrated in FIGS. 23-42. The display system 400 is used to providea visual indication of the terrain advisory and terrain warningindications discussed above as a function of the current position of theaircraft. Background terrain information is also provided which providesan indication of significant terrain relative to the current position ofthe aircraft.

In order to declutter the display, background information may bedisplayed in terms of predetermined dot patterns whose density varies asa function of the elevation of the terrain relative to the altitude ofthe aircraft. Terrain advisory and terrain warning indications may bedisplayed in solid colors, such as yellow and red, relative to thegroundtrack of the aircraft.

An important aspect of the invention is that the terrain backgroundinformation as well as the terrain advisory and warning indications maybe displayed on a navigational or weather display, normally existingwithin an aircraft, which reduces the cost of the system and obviatesthe need for extensive modifications to existing displays, such as anavigational and weather radar type display. In particular, the terraindata is converted to a weather radar format in accordance with thestandard ARINC 708/453 for digital buses on civil aircraft. The terraindata, masked as weather data, can then be easily displayed on anexisting navigational or a dedicated weather radar display that conformswith the ARINC 708/453 serial interface standard.

Referring to FIG. 23, the display system 400 includes a weather radardisplay or navigational display 36 connected to a control bus 404conforming to an ARINC 429 standard and a terrain data bus switch 406.The terrain data bus switch 406 enables selection between terrain dataand weather data for display. More particularly, the data bus switch 406includes a common pole 408 connected to the display 36 by way of aserial display bus 410. One contact 412 on the data bus switch 406 isconnected to a serial weather data bus 414 and, in turn, to a weatherradar R/T unit 416, which transmits weather radar data over the weatherdata bus 414 in conformance with the ARINC 708/453 standard.

An antenna 418 provides weather radar data to the weather radar R/T unit416. The weather radar R/T unit 416 is also connected to the control bus404, which is used for range and mode data.

In accordance with an important aspect of the invention, the terrainadvisory and terrain warning indications, as well as the backgroundinformation discussed above, are converted to "RHO/THETA" format. Theconverted terrain data is then applied to a terrain data serial bus 420,which conforms to the ARINC 453 standard, and connected to a contact 422on the data bus switch 406 to enable selective display of either weatherradar or terrain data on the display 36.

The system for converting the terrain advisory and warning indicationsas well as background terrain data to an ARINC 708/453 standard serialformat is indicated as a functional block 424 in FIG. 23 and isdescribed in more detail below. By converting such data into an ARINC708/453 standard serial format, the data can be displayed on an existingdisplay 36, which conforms to the ARINC 453 standard without the needfor relatively complex and expensive modifications to the display'ssymbol generator.

Referring to FIG. 24, each word 425, in accordance with the ARINC453/708 standard, is 1600 bits long and represents one spoke or radial426 of the display 36. Normally, 512 spokes or radials 426 aretransmitted for each complete sweep of the display 36. At a transmissionrate of, for example, 1 megahertz (MHz), each word 425 takes about 1.6milliseconds (msec) to transmit. For 512 spokes, one complete sweep willthus take about 4 seconds.

The 1600-bit words 425 include a header 428 and 512 range bins 430. Theheader 428 includes the control data on the control bus 404 and alsoincludes a 12-bit code representing the antenna scan angle; the angle ofthe spoke relative to the aircraft heading equivalent to an indicator UPdirection. The range bins 430 cover the distance from the antenna 418 tothe maximum selected. Each of the 512 range bins includes threeintensity bits, encoded into colors as indicated in TABLE 3 below.

                  TABLE 3                                                         ______________________________________                                        3-BIT RANGE CODE COLOR                                                        ______________________________________                                        000              BLACK                                                        100              GREEN                                                        010              YELLOW                                                       110              RED                                                          001              --                                                           101              CYAN                                                         011              MAGENTA                                                      111              --                                                           ______________________________________                                    

As will be discussed in more detail below, the display system 400 iscapable of displaying background terrain information as well as terrainthreat indications as a function of the current position of theaircraft. The threat detection algorithm is run along the groundtrack ofthe aircraft. Thus, terrain threat indications are typically displayedalong the groundtrack while background terrain is displayed relative tothe heading of the aircraft.

Referring to FIG. 25, the terrain background information is shown on thedisplay 36. As will be discussed in more detail below, the elevation ofthe highest terrain relative to the altitude of the aircraft is shown asa series of dot patterns whose density varies as a function of thedistance between the aircraft and the terrain. For example, a relativelydense dot pattern 432 may be used to indicate terrain that is, forexample, 500 feet or less below the aircraft. A medium dense dot pattern434 may be used to represent terrain that is 1000 feet or less below theaircraft, while a lightly dotted pattern 436 may be used to indicateterrain 2000 feet or less below the aircraft. In order to declutter thedisplay 402, terrain more than, for example, 2000 feet below theaircraft, is not shown. The dots may be-displayed in one of the colorsindicated in TABLE 3 above, for example, yellow (amber). In addition, inthe vicinity of an airport, a runway graph 438, for example, in green,may also be provided. The apparent display resolution may be increasedby using more than one color along with the variable density dotpatterns, for example, as shown in TABLE 4 below.

                  TABLE 4                                                         ______________________________________                                        DOT DENSITY HIGH        3000    0                                             PATTERN     MEDIUM      4000    1000                                                      LOW         5000    2000                                          COLOR                   GREEN   YELLOW                                        ______________________________________                                    

The display of a terrain threat indication is illustrated in FIG. 26. Asshown, the terrain threat indication may be displayed contemporaneouslywith the background terrain information. Terrain advisory and terrainwarning indications are displayed in solid shapes 440 and 442,respectively, for example, "squares"; the displayed terrain map cellswhich represent a threat painted solid yellow or red. More particularly,at an aspect ratio for the display of, for example, 3×4 (vertical tohorizontal), the terrain cells will appear "square", particularly atrelatively low-range settings. Colors are used to distinguish betweenterrain advisory and terrain warning indications. For example, red maybe used to represent a terrain warning indication 442 while yellow oramber is used to represent a terrain advisory indication 440. By usingcolored shapes for terrain threat indications and dot patterns ofvariable density for the terrain background information, clutter of thedisplay 402 is minimized.

The terrain data from the terrain data base 24, for example, asillustrated in FIG. 4B, is used to compute the terrain threatindications as well as background information for the display 36. Asdiscussed above, the terrain data base 24 may be stored in a flash ROM444 (FIG. 27) for convenience of updates. In order to facilitate updateof the display 36 as a function of the position of the aircraft, terraindata from the flash ROM 444 is transferred to a random access memory(RAM) A, identified with the reference numeral 446. As will be discussedin more detail below, the RAM A is mapped with terrain data from theflash ROM 444, centered about the current position of the aircraft, asillustrated in FIG. 27.

In order to provide a relatively large display range, for example, 160nautical miles, while minimizing processing time as well as the size ofthe RAM A, the RAM A is configured as a "wedding cake" as generallyillustrated in FIG. 28 and configured such that the highest terrainresolution is near the position of the aircraft and the terrain farthestfrom the aircraft position has the lowest resolution. For example, theRAM A may be provided with four (4) layers. As shown, the instantaneousaircraft position is shown generally centered in layer 1, having thehighest resolution. The resolution of layer 2 lies between theresolution of layer 3 and layer 1. Layer 4 (not shown) is provided witha relatively coarse resolution for use in a cruise mode.

Such a variable resolution display, configured as a layered weddingcake, minimizes the memory size of RAM A as well as the processing time.In order to improve the performance of the system, the flash ROM 444 mayalso be configured similar to a "wedding cake", as generally illustratedin FIG. 27.

As discussed above, the terrain data base 24 may be provided withvariable resolution with a relatively fine resolution of, for example,30×20 arc sec (i.e. 1/2×1/2 arc min). The relatively coarse resolutionof layers 2 and 3 can be formed by combining finer resolution maps. Withsuch a resolution, the configuration of RAM A may be selected inaccordance with TABLE 5.

                  TABLE 5                                                         ______________________________________                                                   RESOLUTION LAYER SIZE                                              LAYER      (MINUTES)  (MINUTES)                                               ______________________________________                                        1          1/2 × 1/2                                                                          32 × 32                                           2          1 × 1                                                                              64 × 64                                           3          2 × 2                                                                              128 × 128                                         4          5 × 5                                                                              REMAINING AREA                                          ______________________________________                                    

Each layer is generally centered relative to the aircraft position asshown in FIG. 28 at the time of the update of RAM A. Each layer of the"wedding cake" has its own update boundary. For example, layer 1 of theRAM A includes an update boundary 448. As the aircraft passes throughthe update boundary 448, the RAM A is updated with new terrain data fromthe flash ROM 444 (FIG. 27) as will be discussed in more detail below.

Because of the refresh rates required for the display 36, the terraindata may be stored in files alternative to the format illustrated inFIG. 4A. For example, the file format of the terrain data may include anindex file or header 55, stored in one or more 128K byte storage blocksand a plurality of data files 51, as generally shown in FIG. 29.

As shown, each index file 55 may include pointers to specific data files51, the length of the map file, as well as the position of one or moreof the corner boundaries of the file 51, for example, northwest andsoutheast corners of each map file 51, which facilitates updates of thedisplay 36 as a function of the current aircraft position. The indexfile 55 may contain information regarding the length of the map file 51,a start of file or altitude offset, the resolution of the map file 51,and whether and how the file 51 is compressed to enable use of variouscompression algorithms.

The data files 51 may include a file header 454 as well as data blocks456 which, as shown, can be grouped according to resolution size. Thefile header 454 indicates various information about the file. The formatof the header 454 may be structured as follows:

<MAJOR VERSION BYTE><MINOR VERSION BYTE><FILE STATUS BYTE><ATTRIB BYTE>

<FILE NAME (8 CHARACTERS)><EXTENSION (4 CHARACTERS)>

<FILE LENGTH LONGWORD>

<TIME STAMP LONGWORD>

<CRC LONGWORD>

<SPME LONGWORD>

The major and minor version bytes relate to the revision status of thedata in the data files 51. The minor version is set as the inverse ofthe byte value of the major version such that a default erase willindicate a zero version. The file status byte 457 provides variousinformation regarding the status of the file as discussed below, whilethe attribute byte is reserved. The file name, extension file length,and spare data are self-explanatory. The time stamp relates to theoperational time of the system (i.e. length of time in use) while theCRC data relates to the data files 51.

The status byte may be used to provide the following informationregarding the data blocks 456, for example, as set forth in TABLE 6below.

                  TABLE 6                                                         ______________________________________                                        FLAG BYTE VALUE INFORMATION                                                   ______________________________________                                        FF              EMPTY                                                         FE              DOWNLOADING                                                   FO              VALID                                                         EO              READY TO ERASE                                                --              RESERVED                                                      --              RESERVED                                                      00              RESERVED                                                      ______________________________________                                    

The EMPTY status (FF) is set by erasing the data blocks 456, while theDOWNLOADING status (FE) is set as part of the image. The VALID status(FO) is set when the file is loaded. Lastly., the READY TO ERASE (EO) isset by the file system for obsolete blocks. The file structure may alsobe as described in Appendix 1.

As indicated below, various corrections of the data files 51 may berequired in areas near runways. FIG. 30 illustrates a portion of theterrain data base 24 around a particular airport, Boeing Field, with aresolution of, for example, 1/2×1/2 minute subcells 455, shown with arunway 456 drawn in. The elevations, as discussed above, are selected tobe the highest elevations within the cells 455, rounded up to the nexthighest resolution increment, for example 100 feet. While such a systemprovides useful and conservative terrain data for use with the terraindata base 24, such a system in the vicinity of an airport can cause theelevations of the runway 456 to be too high or too low. For example, theelevation of the runway 456 at Boeing Field is known to be 17/15 feet;rounded off to zero (0) feet. FIG. 30 shows the elevation of runway 456to be 200/300 feet, for example, by the averaging algorithm discussedabove. Such errors in the runway elevation can degrade systemperformance, and, in particular, cause improper performance of theterrain advisory and terrain warning cut-off boundaries below 500 feet,for example, as illustrated in FIG. 22.

In order to solve this problem, the actual elevation of the runway 456,rounded to the nearest resolution, is used for cells 455 around aperimeter of the runway 456, as generally shown in FIG. 31. Inparticular, a first perimeter, illustrated by the dashed box 458, isselected to be about ±1 cell (i.e. 1/2 minute) from the centerline ofthe runway 456. The elevations in all of the cells 455 intersected bythe dashed box 458 are set to the actual runway elevation and rounded tothe nearest resolution. For Boeing Field, the elevation for such cellsis 0 feet.

Consideration is also given for a narrow approach and a wide approach.The dashed box 460 represents a wide approach while the dashed box 462represents a narrow approach. The cells 464 and 466, intersected by thewide approach box 460, are set on the actual runway elevation, roundedto 0 feet. Inside the approach perimeter, as indicated by the arc 467,the elevation for the cells is selected to be either the runwayelevation or the ΔH terrain floor (as discussed above). For example, ifthe corner of the cell 468, closest to the runway 456, is 1.386 nM fromthe runway 456, the ΔH terrain floor will be 86 feet; rounded to nearest100 feet=100 feet. Thus, the elevation of cell 468 is selected as 100feet.

FIG. 32 illustrates the differences between the elevations in selectedcells 468, 470 and 472 before and after correction. Before correction,the elevation of cells 468, 470 and 472 was 300 feet. After correction,the elevation of cells 470 and 472 are corrected to the runway elevationand rounded to the nearest resolution (i.e. 0 feet). The cell 468 iscorrected to the ΔH terrain floor elevation of 100 feet.

FIG. 33 illustrates another situation where the elevation of cells, dueto the rounding-off method discussed above, can result in a situationwhere the cell elevations adjacent a runway, represented as a staircase474, are below the runway elevation. The correction method, discussedabove, can also be used to correct the various cells around the runway456 up to the runway elevation in a manner as generally discussed above.

FIG. 34 represents a simplified block diagram for implementation for thedisplay system 400 in accordance with the present invention. The displaysystem 400 may include a microprocessor, for example, an Intel type80486, 25 MHz type microprocessor ("486") and an Analog Devices typedigital signal processor (DSP). The DSP is primarily used forcalculating the RHO/THETA conversions to off load the 486.

The display system 400 may have a normal mode and a cruise mode. In anormal mode, the display system 400 displays terrain out to, forexample, 100 nautical miles, while in cruise mode out to about 320nautical miles. In the cruise mode, it is contemplated that thebackground terrain display levels can be lowered by the pilot such thatterrain, for example 10,000 feet or 20,000 feet below the aircraft, canbe displayed. Alternatively, the display level could automatically beadjusted to the highest terrain within the selected display range; thevalue of this level set forth in amber, for example.

The 486 runs a "file picker" routine which searches the terrain database 24 for terrain data covering the area relative to the currentposition of the aircraft. For example, in a normal mode, the file pickerroutine searches the terrain data base 24 for terrain data covering anarea approximately 256×256 minutes relative to the current position ofthe aircraft. As indicated above, the terrain data base 24 includesindex files 55 (FIGS. 4B and 29) which may include the northwest andsoutheast corners of each map file 51 in order to facilitate the search.A list of files covering the area is assembled and maintained.

As indicated above, the index file 55 also includes information as towhether and how the data files 51 are compressed. The files may becompressed/decompressed by a modified Hoffman code, such as PKZIP asmanufactured by PKWARE, INC.

If the data files 51 are compressed, the files are decompressed in thebackground because of the relatively long decompression time and storedin a decompressed file RAM (DFR) 476, for example 256K bytes. Thedecompressed files are then transferred to RAM A, identified with thereference number 446. On power-up, one or two files around the aircraftare decompressed and displayed to allow immediate display of a limitedrange, relative to the position of the aircraft. As more terrain datafiles are decompressed, the additional range is displayed.

The RAM A may be configured in a plurality of cells 479 (FIG. 34),representative of a terrain area 1/2×1/2 minutes, with true northappearing at the top of RAM A as shown. Each cell 479 is two bytes wideand includes data regarding the highest altitude in the cell as well asthe identity of each cell 479. As such, the RAM A is addressable by Xand Y coordinates and thus represents a map with an altitude in eachcell 479. The identity of the cell may be used to distinguish betweenterrain data, for example, from the terrain data base 24 and obstacledata. As discussed above, in some applications, an obstacle data basemay be utilized.

As indicated above, the RAM A may be configured as a wedding cake withvarious layers, as generally shown in FIG. 28 with the top layer (i.e.layer 1) having the highest resolution (i.e. 1/2×1/2 minute cells).Terrain data is loaded into the RAM A such that the aircraft is roughlycentered in the top layer, where the terrain advisory and warningdetection algorithms are run. Once the aircraft moves outside the updateboundary 448 (FIG. 28), the top layer of the RAM A is reloaded withterrain data with the aircraft centered. The lower layers may also beprovided with update boundaries, which enable the lower layers to beupdated independently and less frequently than the top layer.

As discussed above and illustrated in FIG. 27, the flash ROM 444 for theterrain data base 24 may also be configured as a wedding cake asillustrated in FIG. 28. In such an implementation, the highestresolution is provided near the airports while the coarser resolution isprovided away from the airports as generally shown in FIG. 27. Forexample, as illustrated in FIG. 34A, the terrain data base 24 may beconfigured with 1°×1°, 2°×2° or 10°×10° map files. Some of the map filesmay overlap, similar to a "wedding cake". As shown, the terrain mapsclosest to airports are provided with the highest resolution, forexample 15×15 arc seconds. Maps further away from the airport may beprovided with resolutions of, for example, 1×1 minute or 2×2 minuteswhile a coarse resolution (i.e. 2°×2° or 10°×10°) may be provided evenfurther from the airport.

An upload routine may be used to upsample or downsample terrain orobstacle data into the RAM A from the DFR RAM 476 as shown in FIG. 34.Such a routine is well within the ordinary skill in the art. The topthree layers (i.e. layers 1, 2 and 3) of the RAM A (FIG. 28) are updatedfrom the map file source from the flash ROM 444 for the terrain database 24 having the highest resolution, except for the 5×5 minute layer,which is updated only from a 5×5 minute file source to reduce processingtime. Alternatively, as shown in FIG. 34A, an additional RAM A*,identified with the reference numeral 477, may be provided andstructured similar to the RAM A 446, forming a "ping-pong" memoryarrangement. In this embodiment, one terrain data file 51 isdecompressed one file at a time and loaded into the DFR RAM 476 and thenupsampled to either the RAM A or RAM A*. For example, while the RAM A isbeing loaded with additional decompressed files, the terrain threat anddisplay algorithms are executed from the RAM A*. Once the RAM A isupdated, the RAM A is then used for the computations while the other RAMA* is updated with new data from the DFR RAM 476 to account for changesin the aircraft position.

The 5×5 minute resolution layer is primarily intended for use in acruise mode to provide a look-ahead range of about 320 nM. As discussedabove and illustrated in FIG. 29, each file includes an index file 55which contains data relative to the resolution of the terrain data in aparticular map file 51. Such information can be used to vary theresolution of the map files as a function of longitude and latitudeposition. For example, the latitude resolutions of the map files can bedecreased at higher latitudes in a northward direction. Moreover,longitude resolution in the RAM A can be varied and provided with, forexample, a one degree latitude hysteresis to prevent flipping back andforth as an aircraft cruises northwardly. Once the terrain data from theterrain data base 24 is loaded into the RAM A, the terrain advisory andwarning algorithms ("detection algorithms") are run once per second asdiscussed below.

The terrain threat algorithms discussed above are based on a singlevector along the groundtrack. The single vector provides acceptableresults because the relatively crude resolution of the terrain data base24 is based upon the highest elevation per cell and the naturalnoisiness of the groundtrack angle and position information. However,the single vector approach does not take into account various errors,due to, for example, the terrain data base 24, GPS longitude andlatitude errors as well as the published track angle accuracy. As such,the single vector along the groundtrack may be substituted with an arrayof vectors, as generally shown in FIG. 35, which takes such errors intoaccount. In particular, the groundtrack vector is identified with thereference numeral 482 originating at the instantaneous aircraft positionX₀, Y₀. A pair of vectors 484 and 486 are disposed at a configurabledistance D_(OFF), for example 0.2 nM along a segment 488, perpendicularto the vector 482 at points X₁, Y₁ and X_(r), Y_(r). A pair of outsidevectors 490 and 492, originating at points X₁, Y₁ and X_(r), Y_(r),respectively, are directed at a configurable angle D_(ALPHA), relativeto the vectors 484 and 486. The configurable angle D_(ALPHA) may beselected as 3 degrees, equivalent to the groundtrack accuracy of theGPS.

The coordinates X₁, Y₁ and X_(r), Y_(r) for the vectors 490 and 492,respectively, are determined. More particularly, the incremental X and Yvectors, DX_(OFF) and DY_(OFF), are determined as set forth in equations(25) and (26) as set forth below.

    DX.sub.OFF ={D.sub.OFF /COS (LAT)}*COS (GROUNDTRACK),      (25)

where D_(OFF) is a configurable distance between X₀, Y₀ and X₁, Y₁, forexample 0.2 nm and the factor 1/COS (LAT) is used to convert DX_(OFF) tonautical miles when DX_(OFF), DY_(OFF) is in minutes and D_(OFF) is innautical miles.

    DY.sub.OFF =DOFF*SIN (GROUNDTRACK)                         (26)

The coordinates X_(r), Y_(r) can be determined in a similar manner.

The threat detection algorithms are initiated by calculating the rangeincrements DELTA X_(NS) and DELTA Y_(NS) along a north-south coordinatesystem along the groundtrack. Referring to FIG. 36, each spoke 426 has512 size range bins as discussed above, or 256 double range bins. Inorder to convert from an X-Y coordinate system to the north-southcoordinate system, the range increments DELTA X_(NS), DELTA Y_(NS) aredetermined in accordance with equations (27) and (28). (The term 1/COS(LATITUDE) is used to convert from arc minutes to nautical miles.)

    DELTA X.sub.NS =DETECTION STEP SIZE*SIN (GROUNDTRACK)* 1/COS (LATITUDE)(27)

    DELTA Y.sub.NS =DETECTION STEP SIZE*COS (GROUNDTRACK)      (28)

After the range increments DELTA X_(NS) and DELTA Y_(NS) are determined,the threat detection algorithm computation is initiated at theinstantaneous position X₀, Y₀ of the aircraft (FIG. 37). The position isthen incremented by the range increments DELTA X_(NS) and DELTA Y_(NS)and the range is incremented by a configurable step size, for example1/8 unit, as discussed above. After the new coordinates are determined,the current terrain altitude at the new coordinate is determined. Theterrain detection algorithms, as discussed above, are then computed forthe current advisory LAD, as indicated by the arc 494, defining a yellowthreat detection vector 496. In order to provide additional warning timefor a terrain warning, the terrain warning indication can be computedout to a distance equal to 1.5*LAD, as indicated by the arc 498 todefine a red threat detection vector 500. The above steps are repeatedfor the look-ahead vector arrays 490 and 492 (FIG. 35). The entirecomputation is done at a rate of once per second.

Whenever the threat detection algorithm detects terrain along thelook-ahead vector arrays 490 and 492 (FIG. 35), the offending terrain ispainted in an expanded cone around the groundtrack as shown in FIG. 37.In particular, the expanded cone includes a plurality of vectors,spaced, for example, 4° apart (configurable value), for ±90°(configurable value) on each side of the groundtrack. For an expandedterrain warning, the cone may be expanded out distances, for example asset forth in TABLE 7 below.

                                      TABLE 7                                     __________________________________________________________________________    (For an expansion factor of 2)                                                       NORMAL         EXPANDED                                                TYPE OF                                                                              LOOK-AHEAD/    LOOK-AHEAD/                                             THREAT LOOK-DOWN                                                                              LOOK-UP                                                                             LOOK-DOWN                                                                              LOOK-UP                                        __________________________________________________________________________    ADVISORY                                                                             1.0 *    2.0 * 2.0 *    4.0 *                                                 LAD      LAD   LAD      LAD                                            WARNING                                                                              0.5 *    1.5 * 1.0 *    3.0 *                                                 LAD      LAD   LAD      LAD                                            __________________________________________________________________________

The terrain threat is painted on the display 36 by calculating thestarting vector angle DISPA as set forth in equation (29) when a terrainthreat along the groundtrack has been detected.

    DISPA=GROUNDTRACK+30 degrees                               (29)

The range increments are then determined as set forth in equations (30)and (31).

    DELTA X.sub.NS =DISPLAY STEP SIZE*SIN (DISPA)*1/COS(LATITUDE)(30)

    DELTA Y.sub.NS =DISPLAY STEP SIZE*COS (DISPA),             (31)

where the display step size is a configurable constant, for example 1/2unit.

After the range increments DELTA X_(NS) and DELTA Y_(NS) are determined,the instantaneous position of the aircraft X₀, Y₀ is incremented by thesame and the range is incremented by the step size. The altitude is thendetermined for the current position on the terrain map. Subsequently,the threat detection algorithms are run for the distances as discussedabove. Should a threat be detected, a color is loaded into a RAM B,identified with the reference numeral 504 (FIG. 34). At the end of theyellow and red threat vectors 496 and 500, the vector angle DISPA isincremented by 4 degrees with the above steps being repeated until allvectors are done.

In addition to the terrain detection and display algorithms, abackground terrain algorithm is also executed relative to the terraindata in RAM A (FIG. 34) once every screen update, about 4 seconds. Thebackground terrain algorithm simply compares the current altitude of theaircraft with the various terrain altitude data in the RAM A. Any of thedot patterns 432, 434 and 436 (FIG. 25) resulting from the computationare stored in the RAM B (FIG. 34) organized in a wedding-cakeconfiguration (FIG. 28), similar to the RAM A (FIG. 34). In addition,the RAM B is organized in single byte cells containing the color/dotpattern as discussed above. The closest runway may also be stored in theRAM B as either a circular or square pattern in, for example, a greencolor.

The color/dot pattern in the RAM B (FIG. 34) is transferred to the DSPmemory RAM B*, identified with the reference numeral 506, once perscreen update (i.e. 4 sec) and synchronized to the DSP screen paintingperiod. If a threat is detected as discussed above, a new RAM B image istransferred to the DSP RAM B* 206 immediately. The image transfer timetypically fits between two spokes. If not, the DSP will send an oldspoke stored in its spoke memory 508, 510 twice.

The DSP then performs the RHO/THETA conversion on the data in RAM B* 506using the updated-aircraft position, selected range and heading datafrom the 486, as discussed below. Changes in the selected range areimplemented on the fly (i.e. the next spoke) without restarting thesweep.

As discussed above, the ARINC 708/453 weather radar format consists of512 spokes covering a configurable scan range of typically ±90 degreesand a range selected, for example, on a weather data control panel. ForARINC systems, each spoke is calculated twice, once for the leftselected range and once for the right selected range. The spokes arethen transmitted by way of a serial port 512, interleaved left spoke andright spoke to the bus switch 406 (FIG. 23) by way of a pair of drivers514 and 516 (FIG. 34).

The resolution of the scan conversion is 256 range bins; each range binactually being a double range bin per the ARINC 708/453 standard format.Such a resolution is sufficient for the dot patterns 518 illustrated inFIGS. 25 and 26.

The RHO/THETA conversion is best understood with reference to FIG. 38.In particular, once per sweep (i.e. every 4 seconds) the range incrementR1 is determined. The range increment is the selected range divided by256. After the range increment is determined, the starting spoke angleis determined (i.e., the aircraft true heading plus 90 degrees). Lastly,the incremental spoke angle DELTA ALPHA is determined. The incrementalspoke angle DELTA ALPHA is the sweep range divided by 512; the totalnumber of spokes per sweep.

Once per spoke (i.e. 512 times per 4 second sweep), the incrementalrange coordinates DELTA X_(ns), DELTA Y_(ns) are computed and the spokeangle is incremented by DELTA ALPHA, as discussed above. The incrementalrange increment coordinates X_(ns) and Y_(ns) are computed in accordancewith equations (32) and (33), respectively.

    DELTA X.sub.ns =RI*SIN (SPOKE ANGLE)*1COS (LATITUDE)       (32)

    DELTA Y.sub.ns =RI*COS (SPOKE ANGLE)                       (33)

Once per range increment (i.e. 512×256 times per 4 second sweep) , theinstantaneous aircraft position X₀, Y₀ is incremented by DELTA X_(ns)and DELTA Y_(ns). The color/pattern for the new position is looked upand output to the display 36. These steps are repeated for the rest ofthe spoke.

With the use of the DSP, two computations can be done in parallel. Inparticular, the color/pattern of each range bin including the conversionof the longitude to nautical miles (i.e. multiplication by 1/COS (LAT)).With reference to FIG. 38, this computation is performed relative to thetrue north map coordinate system with the aircraft true heading pointingup and the spoke angles referenced to it. The second computationaddresses each display indicator pixel in order to paint the requireddot pattern, relative to the display screen coordinate system.

The generation of dot pattern is illustrated in FIGS. 39-42. In general,the scan conversion is simulated by overlaying each RHO/THETA range binover a calculated indicator pixel map which includes corrections fordisplay aspect ratio using fractal 8×8 dot patterns.

The fractal patterns corresponding to the variable density dot patterns432, 434 and 436 (FIG. 25) are illustrated in FIG. 40. The fractalpattern size is selected to be 8×8 pixels. The densest pattern 432includes 1/2, or 32, of the 64 pixels filled. In the medium densitypattern 434, 16, or 1/4, of the pixels are filled. In the least densepattern 4 of 64, or 1/16, of the pixels are filled.

Referring to FIG. 39, the display screen is divided into a pixel map,consisting of, for example, 256×256 pixels 520. As mentioned above, the8×8 fractal patterns are overlaid onto the pixel map to provide thevariable density dot patterns 432, 434 and 436 illustrated in FIG. 25.

The computation of the dot patterns is best understood with reference toFIGS. 39, 41 and 42. Using an X-Y coordinate system, each spoke isdetermined at the bottom center of the display 36. For a 256×256 pixeldisplay, the starting point corresponds to 120, 0. Next, the incrementalX and Y coordinates relative to the scan angle ALPHA are determinedaccording to equations (34) and (35).

    DELTA X RI*SIN (ALPHA)                                     (34)

    DELTA Y RI*COS (ALPHA),                                    (35)

where RI=a double range increment and ALPHA=the scan angle. Theseincrements are added to the starting point 128, 0. After each increment,the selected fractal pattern is looked up in the RAM B * to determinewhether to send the last range increment as either block or colored.

Referring to FIG. 42, the increments DELTA X and DELTA Y are applied toa pair of adders 522 and 524, which, in turn, are applied to anX-counter 526 and a Y-counter 528. As discussed above, the X counter 526starts at 128 and counts up if DELTA X>0. If DELTA X<0, the X-countercounts down, indicating a pixel on the left (FIG. 39) of the startingposition. Since the starting pixel is at the bottom of the display, theY-counter 528 starts at zero and always counts up. A range counter 528counts up to 256; the maximum number of double range bins. The rangecounter 528 is driven by an increment clock such that the pattern/coloris painted, once per range increment. The output indicates whether thepixel is to be painted left black.

As discussed above, the background terrain algorithm determines theparticular fractal pattern to be used. This data is applied to the DSPto enable the pixels on the pixel map (FIG. 29) to be painted or leftblank. In particular, the three least significant bits from the Xcounter 526 and Y counter 528 are used to select the fractal pattern asshown.

The scan conversion simulation is performed in parallel with theRHO/THETA conversion which determines the color/dot pattern of the rangebin to be transmitted. The color/dot pattern provided by the RHO/THETAconversion is then overlaid with the fractal dot pattern to determinewhether the range bin color should be black or painted to provide theproper dot pattern on the display.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described above.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. A device for alerting a pilot to a hazardouscondition on approach to a runway comprising:an input for receivingsignals representative of a position of the aircraft, a glidepath of theaircraft, and coupled to a data base of stored terrain and airportrunway information; an output; a signal processing device, coupled tosaid input, and coupled to said output, for: (a) selecting from saiddata base of stored terrain and airport runway information, informationpertaining to a runway of intended landing; (b) establishing a first cutoff altitude as a function of runway elevation; (c) establishing asecond cut off altitude as a function of aircraft altitude and distancefrom the runways; (d) selecting between said first and second cut offaltitudes to obtain an absolute cut off altitude, and (e) outputting analert signal as a function of said absolute cut off altitude to alertthe pilot that the aircraft is in danger of impacting terrain prior tolanding on said runway of intended landing.
 2. The apparatus of claim 1wherein said signals representative of the position of an aircraftinclude a first signal received from a satellite navigation systemindicative of an aircraft altitude and a second signal representative ofthe aircraft altitude received from a source other than the satellitenavigation system, and wherein said signal processing device furthercomprises a means for determining a compound altitude signal.
 3. Theapparatus of claim 1 wherein said signal processing device comprises amicroprocessor.
 4. The apparatus of claim 1 wherein said signalprocessing device comprises a means for outputting said alert signal asa video control signal, wherein said video control signal is useful forcontrolling a cockpit visual display.
 5. The apparatus of claim 1further comprising a voice warning generator coupled to said output andwherein said alert signal output from said signal processing devicecomprises an audio control signal to command said voice warninggenerator to ascertain aural alert.
 6. The apparatus of claim 1 whereinsaid absolute cut off altitude is set to a fixed altitude when theaircraft is located at a pre-determined distance from the runway.
 7. Amethod for alerting a pilot of an aircraft to a hazardous condition onapproach to a runway, comprising the steps of:receiving signalsrepresentative of a position of the aircraft, a glidepath of theaircraft, and terrain and airport runway information; selecting fromsaid terrain and airport runway information, information pertaining to arunway of intended landing; establishing a first cut off altitude as afunction of runway elevation; establishing a second cut off altitude asa function of aircraft altitude and distance from the runway; selectingbetween said first and second cut off altitudes to obtain an absolutecut off altitude; and outputting an alert signal as a functioning saidabsolute cut off altitude to alert the pilot that the aircraft is indanger of impacting terrain prior to landing on said runway of intendedlanding.
 8. The method of claim 7 wherein said step of outputting analert signal further comprises the step of outputting a video controlsignal to control a cockpit visual display.
 9. The method of claim 7further comprising the step of receiving a first and second altitudesignals from a first and second distinct source respectively to obtain acompound altitude signal representative of the aircraft altitude. 10.The method of claim 7 wherein said step of outputting an alert signalcomprises outputting an audio control signal to generate an aural alert.